Ventilated cask for nuclear waste storage

ABSTRACT

A natural passively cooled ventilated cask includes a cavity which holds a canister containing heat and radiation emitting spent nuclear fuel assemblies or other high level wastes. Ambient ventilation or cooling air is drawn inwards beneath the cask and vertically upwards into a lower portion of the cavity through air inlet ducts formed integrally with a bottom canister support structure coupled to the cask. The air heated by the canister flows upwards in the cavity and returns to atmosphere through air outlet ducts in the cask lid. Air circulation is driven via natural convective thermo-siphon flow. Structural standoff members elevate the bottom of the cask above a concrete base pad forming an air inlet plenum beneath the canister support structure. The lateral sidewall surface of the cask has no penetrations for the air inlets, which eliminates any streaming path for radiation emanating from the spent nuclear fuel.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 17/470,053 filed Sep. 9, 2021, which claims the benefit of U.S.Provisional Application No. 63/211,234 filed Jun. 16, 2021 and U.S.Provisional Application No. 63/223,578 filed Jul. 20, 2021; which areall incorporated herein by reference in their entireties.

BACKGROUND

The present invention relates generally to ventilated overpacks or casksused for dry storage and/or transport of high level nuclear waste fromnuclear power generating plants or other nuclear facilities.

In the operation of nuclear reactors, the nuclear energy source istypically in the form of a plurality of hollow Zircaloy tubes eachfilled with enriched uranium pellets, which are collectively arranged inassemblages referred to as fuel assemblies. When the energy in the fuelassembly has been depleted to a certain predetermined level, the fuelassembly is removed from the nuclear reactor and referred to as used orspent nuclear fuel (“SNF”). The standard structure used to package orstore the SNF assemblies discharged from light water reactors foroff-site shipment or on-site dry storage is an all-welded stainlesssteel container. Such containers are well known and may be variouslyreferred to as multi-purpose canisters (MPCs) such as those availablefrom Holtec International of Camden, New Jersey, or dry storagecanisters (DSCs).

Regardless of their name, these SNF canisters are characterized by arelatively thin-walled stainless shell to effectively transmit heatemitted by the decaying the SNF assemblies across the canister's wallboundary. The stainless steel shell has several full through-thicknesscontinuous seam welds, including longitudinal seam welds and girth weldssuch as those that connect the shell to the top and bottom end closureplates. A fuel basket is typically arranged inside a metallic storagecanister which defines an array of prismatic-shaped storage cells eachof which is sized to hold a single fuel assembly, which in turncomprises a plurality of individual spent nuclear fuel rods.

A single canister is in turn stored and enclosed inside its own outervertically ventilated module referred to as an overpack or cask. Thecasks are heavy radiation shielded vessels which block gamma and neutronradiation emitted from the SNF assemblies which passes through thecanister shell and end plates. The ventilated casks are used for safetransport and/or storage of the multiple spent fuel assemblies withinthe inner fuel basket.

In addition to emitting neutron and gamma radiation requiring protectiveshielding, the highly radioactive SNF in the fuel assemblies (or otherhigh level nuclear waste which may be stored in the waste canister)still produces considerable heat which must be dissipated to avoiddamage to the fuel assemblies and spent fuel cladding stored in thecanister. Ventilated casks use available ambient ventilation air to coolthe canister and remove the heat emitted therefrom to protect the fuelassembly. Some casks have lateral ventilation openings in the sidesand/or the lids which can create a path for radiation streaming to theambient environment in some cases.

Improvements in ventilated casks used to store high level nuclear wasteare therefore desired.

BRIEF SUMMARY

An improved nuclear waste storage system comprising a radiation-shieldedventilated cask for storing a nuclear fuel canister therein is provided.In one embodiment, the lateral side surfaces of the cask body does nothave penetrations or openings for air ventilation, which eliminates anypotential lateral or radial streaming paths for radiation emanating fromthe spent nuclear fuel (SNF) or other high level radioactive wastestored inside the canister in the cask. To achieve this, the presentcask in one embodiment comprises a unique concrete-filled canistersupport structure at bottom. This radiation-shielded bottom closurestructure supports the canister and is configured with integral inletducts which draw ambient cooling air into the internal cavity of thecask through downwardly open air inlets in the bottom baseplate of thecanister support structure via a natural convective thermo-siphon floweffect. The canister support structure is configured to elevate andspace the baseplate and concomitantly the cask above the concrete basepad on which the cask is positioned during use. This allows the coolingair to be drawn radially/horizontally inwards beneath the cask and thenflow upwards through the air inlet in a vertical axial directionparallel to the vertical longitudinal or centerline axis of the cask. Anair inlet plenum is formed between the base pad and baseplate of thecask.

Cooling air drawn into the cask continues to rise upward in aventilation annulus formed in the internal cavity between the cask bodyand canister as the air is heated by the hot canister. The heated air isreturned back to the ambient environment through air outlet ductsintegrally formed in the radiation-shielded top lid of the cask. The lidis mounted on the cask body and configured such that there is nomacro-path for leakage of radiation in the lateral or radial directionto the environment. The cask is configured for above-grade mounting inone embodiment so that ambient cooling air can readily reach the airinlet plenum from 360 degrees around the cask and becomes mixed beforeentering the cask. This plenum nullifies any pressure differentialssurrounding the case cause by directional wind flow which enhance equalcooling of the entire canister inside the cask.

The downwardly open inlet ducts in the canister support structure arecircuitously configured to eliminate any downwards straight line ofsight through the ducts to prevent outward straight line radiationstreaming from inside the cask. In one non-limiting embodiment, the airinlet ducts may have a generally Z-shaped transverse cross-sectionalshape. In the unlikely event any radiation stream path were to occur,the straight-line radiation streaming would still be directed downwardsfrom the air inlet towards the thick concrete base pad on which the caskis seated rather than in a radial or lateral direction where nuclearstorage site workers might be present or near a boundary or fencedperimeter of the nuclear waste storage facility. The concrete base padand soil below on which the pads rest are effective radiation shieldingmaterials which act to block or absorb any escaping downwardly-directedradiation streaming.

The air outlet ducts in the cask lid are structured and circuitouslyconfigured as well to eliminate both upwards straight line radiationstreaming and any laterally open air outlet penetrations in the side ofthe lid. Instead, the heated air is discharged back to atmospherethrough the top of the lid via a protective cap structure.

In one aspect, a passively ventilated nuclear waste storage caskcomprises: an elongated cask body defining a top, a bottom, a sidewall,and an internal cavity extending between the top and bottom along avertical centerline axis of the cask, the internal cavity beingconfigured for holding a nuclear waste storage canister; a lid attachedto the top of the cask body; an air outlet formed in the lid; adownwardly open air inlet formed in the bottom of the cask body in fluidcommunication with the internal cavity and ambient atmosphere; and acanister support structure configured to support the canister in theinternal cavity and engage a base pad, the canister support structurefurther configured elevate the bottom of the cask body above the basepad; wherein ambient cooling air is drawn in a flow path radiallyinwards beneath the bottom of the cask body and vertically upwards intothe internal cavity through the air inlet.

According to another aspect, a passively ventilated nuclear wastestorage system comprises: an elongated cask defining a top, a bottom, asidewall, and an internal cavity extending between the top and bottomalong a vertical centerline axis of the cask; a canister configured forholding nuclear waste, the canister positioned in the internal cavity ofthe cask and forming a circumferentially-extending ventilation annulusbetween the canister and cask; a lid attached to the top of the cask andcomprising an air outlet in fluid communication with the internal cavityand ambient atmosphere; a concrete filled canister support structuredisposed at the bottom of the cask and supporting the canister; aplurality of standoff members protruding downwards from the canistersupport structure which support and elevate the bottom of the cask froma base pad to form an air inlet plenum in fluid communication withambient air; and air inlet ductwork formed through the canister supportstructure and in fluid communication with the ventilation annulus andair inlet plenum; wherein the air inlet ductwork is configured to drawambient cooling air radially inwards into the air inlet plenum and theventilation annulus via natural thermo-siphon effect driven by heatemitted from the canister.

According to another aspect, a method for operating a passivelyventilated nuclear fuel storage system comprises: providing a caskcomprising an internal cavity and an ambient air ventilation system influid communication with the internal cavity and atmosphere; inserting acanister containing nuclear waste emitting heat into the internal cavityof a cask and onto a canister support structure fixedly coupled to thecask; drawing ambient cooling air into an air inlet plenum formedbeneath the cask; flowing the cooling air upwards from the air inletplenum through the canister support structure into the internal cavityof the cask; heating the cooling air in the internal cavity of the cask;and discharging the heated cooling air to atmosphere through a lidcoupled to a top of the cask.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein likeelements are labeled similarly and in which:

FIG. 1 is a top perspective view of a passively ventilated cask with forstoring high level nuclear radioactive waste material such as spentnuclear fuel;

FIG. 2 is a bottom perspective view thereof;

FIG. 3 is a vertical partial cross-sectional perspective view thereof;

FIG. 4 is an enlarged detail taken from FIG. 3 ;

FIG. 5 is a top perspective view of a bottom canister support structureof the cask;

FIG. 6 is a top exploded perspective view of the cask;

FIG. 7 is a bottom exploded perspective view of the cask;

FIG. 8 is a side view of the cask;

FIG. 9 is a top view of the cask;

FIG. 10 is a full vertical cross-sectional perspective view of the cask;

FIG. 11 is a full vertical side cross-sectional perspective view of thecask;

FIG. 12 is a partial vertical cross-sectional view of the upper portionof the cask and lid;

FIG. 13 is a transverse/horizontal cross sectional view of the canistersupport structure of the cask;

FIG. 14 is a vertical partial cross-sectional perspective view of the ofthe cask with concrete liner of the sidewall not shown;

FIG. 15 is a vertical partial cross-sectional view of the lower portionof the cask and concrete base pad supporting the cask;

FIG. 16 is a vertical partial cross-sectional perspective view of the ofthe cask with concrete liner of the sidewall and bottom canister supportstructure not shown;

FIG. 17 is a bottom perspective view of the bottom of the cask showinginstalled curved radiation photon scattering attenuators;

FIG. 18 is a top perspective view of the upper portion of the caskshowing a radiation photon scattering duct attenuator installed in theair outlet duct of the ambient air ventilation system of the cask;

FIG. 19 is another vertical cross-sectional view of the cask with aspent nuclear fuel canister positioned in the cask;

FIG. 20 is a bottom view of the cask;

FIG. 21 is an enlarged vertical cross sectional view of an air inletduct of the canister support structure;

FIG. 22 is a top perspective view of the air inlet duct; and

FIG. 23 is a bottom perspective view of the air inlet duct.

All drawings are schematic and not necessarily to scale. Features shownnumbered in certain figures are the same features which may appearun-numbered in other figures unless noted otherwise herein.

DETAILED DESCRIPTION

The features and benefits of the invention are illustrated and describedherein by reference to exemplary embodiments. This description ofexemplary embodiments is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. Accordingly, the disclosure expressly should not belimited to such exemplary embodiments illustrating some possiblenon-limiting combination of features that may exist alone or in othercombinations of features.

In the description of embodiments disclosed herein, any reference todirection or orientation is merely intended for convenience ofdescription and is not intended in any way to limit the scope of thepresent invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivatives thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise.

As used throughout, any ranges disclosed herein are used as shorthandfor describing each and every value that is within the range. Any valuewithin the range can be selected as the terminus of the range. Inaddition, all references cited herein are hereby incorporated byreference in their entireties. In the event of a conflict in adefinition in the present disclosure and that of a cited reference, thepresent disclosure controls.

FIGS. 1-23 depict various aspects and components of a nuclear wastestorage system comprising a first embodiment of a passively cooled andnaturally ventilated outer nuclear waste storage module or cask 100configured for holding a nuclear waste canister 101 which contains spentnuclear fuel (SNF) or other high level radioactive waste materials. Cask100 is constructed and configured for above grade placement in oneembodiment such as on a top surface 131 of a flat reinforced concretebase pad 130 slab which may rest on soil or another structure. The caskmay be anchored to the base pad if desired via fasteners to preventshifting during a postulated seismic event or projectile impact.

Canister 101 is a vertically elongated and hermetically sealed (i.e. gastight) vessel in one embodiment comprising cylindrical shell 103,circular bottom closure plate 104 affixed to a bottom end of the shell,and a circular top closure plate 106 coupled to a top end of the shell.The top and bottom closure plates may be hermetically seal welded to theshell via circumferentially continuous girth seal welds at the weldseams. An interior space 105 is defined within the shell which isconfigured for holding the SNF fuel assemblies 102 (representedschematically in FIG. 11 by dashed lines) or other high levelradioactive waste materials. Such fuel assemblies are well known in theart without further elaboration. For example, a typical rectangular fuelassembly for U.S. style pressurized water reactors (PWRs) is disclosedin commonly-owned U.S. patent application Ser. No. 17/115,005, which isincorporated herein by reference in its entirety. The fuel assemblies102 contain the fuel rods or “cladding” with decaying uranium whichgenerates considerable heat that must be dissipated to protect thestructural integrity and containment of the fuel. This in turn heats thecanister which emits the heat. The canister including shell 103 and thetop and bottom closure plates 104, 106 preferably may be made ofstainless steel in one embodiment for corrosion protection.

Cask 100 may be a heavily radiation-shielded double-walled vessel in oneembodiment including a vertically elongated cask body 110 formed by acylindrical outer shell 111 and inner shell 112, and radiation shieldingmaterial 113 disposed in an annular space 113 a formed therebetween. Theshells 111, 112 and shielding material 113 collectively define thesidewall 110 a of the cask. The inner and outer shells areconcentrically arranged relative to each other as shown.

In one embodiment, the shielding material 113 of cask body 110 maycomprise a high-density concrete mass or liner for neutron and gammaradiation blocking. The concrete in some embodiments may comprisehematite (iron oxide compound Fe₂O₃) and/or other iron-containingaggregates which substantially boost the thermal conductivity of theliners resulting in a significant increase in the rate of conductiveheat transfer from the inside surface of the cask to the outside surfaceexposed to ambient atmosphere. Other radiation shielding materials maybe used in addition to and/or instead of concrete including lead forgamma radiation shielding, boron containing materials for neutronblocking (e.g. Metamic®, Holtite® or others), steel, and/or othersshielding material typically used for such purposes in the art.

Inner shell 112 of the cask body 110 defines an interior or internalsurface 112 a and outer shell 111 defines an exterior or externalsurface 111 a of the cask. Surfaces 111 a, 112 a formed by the shellsmay correspondingly be cylindrical and arcuately curved in oneembodiment. The cask body further includes an open top 119 (when notclosed by lid 114) defined by the upper end of the cask body sidewall110 a and bottom 120 defined by baseplate 115 at the lower end of thecask body sidewall.

The passively cooled and ventilated nuclear waste storage cask 100 maybe vertically elongated and oriented when in use as shown in theillustrated embodiment. The inner and outer shells 112, 111 may beformed of a suitable metallic material, such as without limitation steel(e.g. carbon or stainless steel). If carbon steel is used at least theexterior surface 111 a of the cask may be epoxy painted/coated forcorrosion protection. The metal shells 111, 112 may each haverepresentative thickness of about ¾ inches as one non-limiting example;however, other suitable thicknesses may be used.

Cask 100 comprises a vertically-extending internal cavity 121 whichextends along a vertical longitudinal or centerline axis CL defined byand passing through the geometric centerline of the elongated cask.Cavity 121 may be of cylindrical configuration in one embodiment with acircular cross-sectional shape; however, other shaped cavities withcorresponding cross-sectional shapes may be used including polygonalshapes and other non-polygonal shapes (e.g. rectilinear, hexagon,octagonal, etc.).

Cask 100 further includes a radiation shielded canister supportstructure 140 disposed at the bottom 120 of the cask body 110 whichessentially closes the bottom end of internal cavity 121 of the cask(except for the cooling air inlet structure further described herein toallow ambient ventilation air to enter the cavity at bottom). Thecanister support structure 140 is configured to support and elevate thecanister 101 within the internal cavity of the cask in the manner shown.Canister support structure 140 has a composite construction whichcomprises lower and outermost baseplate 115, upper top plate 141 facingand disposed inside the internal cavity 121 at bottom, peripheralcylindrical vertical shell 144 extending therebetween, and concreteliner 142 for enhanced radiation shielding filling the interior spacecreated by the shell and the top and bottom plates. Top plate 141 andbaseplate 115 may be circular and flat. Baseplate 115 projects radiallyoutwards beyond the vertical shell 144 (see, e.g., FIGS. 4-5 ). Thebaseplate covers the entire bottom of cask body 110 and extendscircumferentially along the entire bottom end of the outer shell 111 ofthe cask body at the perimeter of the cask. The baseplate 115, top plate141, and vertical shell 144 of the canister support structure may bewelded together to form a permanently joined assemblage. Multiple fillholes 145 are provided in top plate 141 to pour the concrete liner intoeach quadrant of the assemblage after welding which is defined by thecanister support rib plates 143 of canister support structure 140 asfurther described herein.

Baseplate 115 may be hermetically seal welded to the bottom ends of theinner and outer shells 112, 111 at the bottom 120 of the cask body andstructurally supports the shells. Top plate 141 has a smaller diameterthan baseplate 115 and is disposed and elevated inside the cask internalcavity 121 spaced upwards from the bottom of the cask. Top plate 141 andbaseplate 115 may each have a flat and circular configuration in onenon-limiting embodiment as shown. The top plate includes a flat upwardfacing top surface 141 a which is exposed inside the cask internalcavity 121. Baseplate 115, top plate 141, and shell 144 of canistersupport structure 140 may be made of a similar metallic material as theshells 111, 112 (e.g., steel or stainless steel). In one embodiment,baseplate 115 may be about 3 inches thick. The bottom surface 115 a ofbaseplate 115 defines the bottom of the cask 100.

The canister support structure 140 forms a pedestal for supportingcanister 101 in cask internal cavity 121 and further comprises anassemblage of multiple intersecting canister support rib plates 143which are an integral part of the support structure. In one embodiment,canister 101 is supported by and seated directly on the rib plates Ribplates 143 are elongated in length and extend radially between opposingdiametrically opposite portions of the sidewall 110 a of the cask body110. The rib plates are embedded in the sidewall 110 a (includingconcrete liner 113) and have vertical outer ends 143 a which may bewelded to outer shell 111 of the cask. Ends 143 a may protrudecompletely through complementary configured slots 143 b in the outershell in one embodiment for additional structural support by the shellbase material in addition to the weldments thereto.

The horizontal bottom edges 143 c of rib plates 143 may be welded to thetop surface of the baseplate 115. Rib plates 143 protrude upwards abovethe top surface 141 a of top plate 141 of the canister support structuresuch that their horizontal top edges 143 d are spaced apart abovesurface 141 a of the top plate to engage the bottom closure plate 104 ofcanister 101 seated thereon. This elevates the canister and forms a gapor space between canister bottom closure plate 104 and top surface 141 aof top plate 141 which allows heat emitted from the bottom of canister101 to be drawn away by the ambient cooling air flowing through internalcavity 121 of the cask 100.

Rib plates 143 in one embodiment may further include a stepped shoulder143 a formed in top edge 143 d of the plates adjacent inner shell 112inside the cask internal cavity 121. Shoulders 143 a engage the sides ofthe canister shell 103 to center and stabilize the canister in the caskduring transport and in the case of a seismic event. In one embodiment,the shoulders 143 a are preferably configured to space the canistershell 103 apart from the inner surface 112 a of the cask inner shell 112such that a substantially uniform ventilation annulus 122 (furtherdescribed herein) is maintained between the cask 100 and canister whenthe canister is positioned in internal cavity 121 of the cask.

In one embodiment, rib plates 143 may be arranged in an “X” shapedintersecting pattern as shown. The intersection of the plates 143 mayfall on the vertical longitudinal or centerline axis CL of the cask. Ribplates 143 may be welded to each other at the intersection in someembodiments. Such an intersecting arrangement strengthens the ribassemblage particularly since the central portion of rib plates 143 areembedded in the concrete liner 142 of the canister support structure 140(see, e.g., FIG. 5 ).

Cavity 121 of cask 100 has a configuration in transverse cross-sectionalarea (perpendicular to centerline axis CL) and height suitable forholding a single SNF canister 101 therein which holds the SNF assembliesor other high level radioactive waste. The internal diameter of caskcavity 121 is intentionally larger than the diameter of the fuelcanister 101 to form a ventilation annulus 122 between the canister 101and inner shell 112 when the canister is emplaced in the cask. Theradial width of annulus 122 preferably is sufficient to draw heatgenerated by the nuclear waste within the canister away from thecanister as the cooling ventilation air flowing upwards alongside thecanister. The upward flow of cooling air in the annulus is automaticallydriven via a natural convective thermo-siphon effect as the air becomesheated by the canister 101. A typical airflow annulus may be in therange of about and including 2-8 inches in radial width as anon-limiting example depending on the estimated heat load of the fuelcanister 100 and ventilation cooling air flow needed. The air flowrateis selected to maintain the desired canister maximum temperature limit.It bears noting that the canister support rib plates 143 maintain theannulus 122 even during occurrence of a seismic event to ensure coolingair can continue to reach the canister.

The ventilation annulus 122 extends vertically for at least the fullheight of the canister and preferably slightly above. The upper portionof canister 101 is laterally/radially supported and centered by aplurality of upper guide members 117 discussed below. Accordingly thecanister 101 has a height approaching the full height of the cask cavity121, and at least greater than ¾ the height of the cavity. Annulus 122further extends all the way down to the top plate 141 of the canistersupport structure 140 and is in fluid communication with the air inlet200 of the cask in structure 140, as further described herein.

The radially and vertically extending guide members 117 are disposed inthe upper portion of the cask cavity 121. The array of upper guidemembers 117 are circumferentially spaced apart and rigidly attached tothe interior/internal surface 112 a of the inner shell 112 such as viawelding. Guide members 117 may be formed of steel plates (e.g., lugs) ortubes, and are provided around the entire inner shell at least at theupper portion in cavity 121 for full 360 degree coverage. The inwardvertical sides or edges of the guide members are configured toabuttingly engage and center the canister 101, which prevents excessivelaterally/radially movement and rattling thereof if vibrated during aseismic event, transport, or when being lifted/lowered by a crane orhoist from or into the cask internal cavity 121. Notably, the guidemembers 117 further act to maintain the ventilation annulus 122 betweencanister 100 and inner shell 112 in the upper portion of the cask cavity121 to preserve this airflow passage for removing heat emitted by thecanister. This ensures that a continuous available flow of ambientcooling can to circulate around and flow upwards along the sides of thecanister for cooling.

A radiation-shielded lid 114 is detachably coupled to the top 119 ofcask body 110 and closes the normally upwardly open cavity 121 of cask100 when in place (except for the cooling/ventilation air outlet paththrough the lid). Air outlet 220 of the cask ventilation system isformed through the lid. The lid 114 may have a “disk and donut”configuration in one embodiment comprising an upper annular member 114 adefining a central air outlet opening 114 f and lower disk member 114 brigidly coupled thereto. Disk member 114 b is sized to fit inside thetop end of cask internal cavity 121. Annular lid member 114 a rests onthe top 119 of the cask body 110 on sidewall 110 a when the lid is inplace. More specifically, lid member 114 a is seated on annular topclosure plate 119 a which is fixedly welded to the top ends of inner andouter shells 112, 111. Multiple fill holes 145 may be formed in closureplate 119 a for pouring the concrete liner 113 between the inner andouter shells 112, 111 of radiation shielded cask body 110. Lid member114 a is detachably coupled to cask body 110 via a plurality of threadedfasteners 146 each secured to a vertical steel lifting plate 146embedded in the concrete liner 113 at the top of the bask body andwelded to the inner and outer shells 112, 111 (see, e.g., FIGS. 12 and14 ).

Lid 114 comprised of the annular and disk members 114 a, 114 b is acomposite structure comprising a hollow metal outer housing 114 cdefining an interior space filled with a radiation shielding material114 d such as a high-density concrete liner encased by the outerhousing. Other shielding materials may be used in addition to or insteadof concrete. Lid 114 provides radiation shielding in the vertical upwarddirection, whereas the concrete liner 113 disposed between the inner andouter shells 112, 111 provides radiation shielding in the lateral orradial direction. With exception of the concrete liner, the housing 114c is preferably formed of a metal such as without limitation steel (e.g.carbon or stainless) capable of withstanding projectile impacts or othersources of potential damage.

According to one aspect of the nuclear waste fuel storage system, thevertical ventilated nuclear fuel storage cask 100 includes a naturalcirculation cooling air ventilation system (i.e. unpowered byfans/blowers) for removing decay heat emitted from the canister 101which holds the SNF or other high level nuclear waste material. Thecooling airflow provided by ambient air surrounding the cask is drivenby a natural convective thermo-siphon effect in which air within theventilation annulus 122 is heated by the canister 101 (containing thedecaying SNF or other nuclear waste material) causing an upflow whichdraws ambient cooling air into the cask at bottom through the air inlet200 in the canister support structure 140.

The cask cooling air ventilation system will now further be described infurther detail.

Referring generally to FIGS. 1-23 as applicable, the cask ventilationprovisions include a cooling air inlet 200 formed in the bottom of cask200 through canister support structure 140 and cooling air outlet 220formed through the lid. Air inlet and outlet 200, 220 are in fluidcommunication with ambient atmosphere and the internal cavity 121 ofcask 100, and more particularly ventilation annulus 122 formed when thenuclear waste canister 101 is in position in the cavity.

The air inlet 200 comprises downwardly open air inlet openings 201formed through horizontal flat baseplate 115 of the canister supportstructure 140 which is affixed to the bottom of the cask body 110. Airinlet openings 201 provide the fluid connection of the lower portion ofcask internal cavity 121 to ambient atmosphere via the air inlet in thecanister support structure. Air inlet openings 201 may be arranged in acircular array in one embodiment which is concentrically and coaxiallyaligned with the vertical centerline axis CL of the cask.

Air inlet 200 comprises inlet ductwork 208 collectively defined by aplurality of compound shaped air inlet ducts 202 formed within andextending through be canister support structure 140 including concreteliner 142. The air inlet ductwork of the ventilated cask 100 thereforedoes not penetrate the sidewall of the cask (i.e. inner shell 112, outershell 111, and concrete liner 113) to prevent radiation streaming in theradial/lateral direction to the ambient environment where workers may bepresent. In the non-limiting illustrated embodiment, eight inlet ducts202 are provided (two located within each quadrant of the cask and itsinternal cavity 121 (see, e.g., FIG. 13 ). Other numbers of inlet ductshowever may be provided in other embodiments and does not limit theinvention. The inlet ducts 202 are preferably arranged in a tightlyspaced circular or annular array or grouping such that the lateralspacing between adjacent ducts is less than the lateral width of eachduct. This tight grouping beneficially ensures that the cylindricalshell 103 of canister 101 is evenly and adequately cooled by acircumferentially-extending curtain of ambient cooling air, as furtherdescribed herein.

The individual inlet ducts 202 of the inlet ductwork 208 in canistersupport structure 140 each have a compound configuration designed suchthat no straight line of sight exists between the internal cavity 121 ofcask 100 (including ventilation annulus 122) and the air inlet openings201 in the baseplate 115 or ambient environment. This prevents straightline radiation streaming in the downwards direction through the airinlet ducts.

Accordingly, in one embodiment, each air inlet duct 202 may include ahollow metal body having generally Z-shaped transverse cross-sectionalshape formed by, in operable fluid communication: a vertical lowerentrance portion 203 fluidly coupled to a respective air inlet opening201 in baseplate 115; a vertical upper exit portion 204 fluidly coupledto the ventilation annulus 122 in the internal cavity 121 of the caskbody 110; and a radial/horizontal intermediate portion 205 fluidlycoupled between the entrance and exit portions. Lower entrance portion203 of each inlet duct 202 is located radially inwardly from itsrespective upper exit portion 204 and therefore is closer to thevertical centerline axis CL of the cask body. Inlet ducts 202 extendthrough the concrete liner 142 of the canister support structure 140 inthe zig-zag shape shown and may be formed of steel or another suitablemetal. The ducts 202 may be welded to the baseplate 115 and top plate141 of the canister support structure before concrete is filledtherebetween embedding the ducts in the concrete.

The upper exit portions 204 of air inlet ducts 202 each define anupwardly discharging air exit opening 206 which discharges andintroduces ambient cooling air passing through the ducts into the bottomof internal cavity 121 of cask 100, and more particularly the bottom ofthe ventilation annulus 122. The upper exit portions 204 and exitopenings 206 are therefore located adjacent to sidewall 110 a of thecask body 110 and at the bottom of the ventilation annulus. In oneembodiment, ducts 202 are arranged as previously described herein suchthat their corresponding air exit openings 206 of the ductsconcomitantly form a circular or annular array openings adjacent to caskinner shell 112 which in turn is concentrically aligned with thevertical centerline axis CL of the cask (see, e.g., FIGS. 4 and 13 ).Advantageously, ambient cooling air is discharged and introduced axiallyand vertically upwards into the ventilation annulus (parallel tocenterline axis CL) to form a circumferentially-extending cooling aircurtain surrounding the canister for 360 degrees. Each quadrant of thecanister is therefore equally cooled in a balanced manner to minimize orprevent localized “hot spots” on the canister shell. It bears notingthat the very small gaps between adjacent air inlet ducts 202 as shownin the figures have no practical effect on effectiveness of cooling thecanister since the incoming ambient cooling air between the ducts mixesupon entry into the bottom of ventilation annulus 122.

It is further notable that introducing the ambient cooling air in thevertically and axially upwards direction into the ventilation annulus122 of cask 100 (parallel to cask centerline axis CL) as opposed tointroducing the air radially into the annulus through the sidewall ofthe cask 100 provides additional benefits. The present cooling airvertical introduction path in which air flows upwards parallel to thesides of the canister (and cask vertical centerline axis CL) enhancescanister cooling since it creates less turbulence when compared toradial introduction of cooling air into the cask which strikes thecanister perpendicularly and disperses. The translates into reducedairflow resistance and a greater naturally driven convective cooling airflow rate (CFM-cubic feet per minute) via the thermo-siphon effect tocool the canister 101 and fuel assemblies therein. This is notableparticularly since the passive cooling air flow is unassisted by a fanor blower. Furthermore, the vertically upwards introduction of ambientcooling air into the cask internal cavity 121 flows in the samedirection as the air heated by the canister which naturally rises andflows upwards via the natural thereto-siphon effect.

According to another aspect of the cask ventilation cooling system, cask100 is raised or elevated above the concrete base pad 130 by a pluralityof standoff members 150 protruding downwards from baseplate 115 toengage the pad. Standoff members 150 collectively support the entireweight the cask 100 and canister 101 with fuel assemblies loadedtherein. In one embodiment, standoff members 150 may comprise steelplates of an elongated rectangular form as shown. However, other shapedstandoff members may be used. To evenly distribute the weight the loadedcask 100, the standoff members 150 may comprise a circular outer arrayof outer standoff members 150 a and a circular inner array of innerstandoff members 150 b. Standoff members 150 a, 150 b arecircumferentially spaced apart with respect to the other standoffmembers in each of their respective arrays. Outer standoff members 150 aare spaced radially outwards from the inner standoff members 150 b onthe bottom baseplate 115. The circumferential spacing allows ambientcooling air to be drawn radially inwards beneath the baseplate 115 ofthe cask and upwards into the circular array of air inlet openings 201formed therein. In one embodiment, the inner standoff members 150 b maybe located closer to centerline axis CL of cask 100 than and inside ofthe air inlet openings 201.

With respect to the natural thermo-siphon cooling air ventilationsystem, standoff members 150 form an air inlet plenum 151 beneath thecask 100 due to vertical spacing or gap between baseplate 115 and theconcrete base pad 130. The standoff members 150 are spaced apart asnoted above which plays the important function of allowing ambient airto enter the plenum and reach the air inlet openings 201 of the airinlet ducts 202. An inflow of ambient air enters the plenum 151 from afull 360 degrees around the entire bottom perimeter of the cask 100.Cooling air is drawn through/between standoff members 150 into theplenum and then enters internal cavity 121 of the cask via the air inletopenings 201 of the ducts 202 formed in the baseplate 115.

Advantageously, because the inlet openings 201 are formed as cuts oropenings in the flat horizontal baseplate 115 of canister supportstructure 140, the inlets are relatively insensitive to the direction ofthe wind since the ambient air first collects in the air inlet plenum151 formed beneath the baseplate 115 before vertically rising andentering the air inlet openings. This contrasts to designs in whichventilation air inlet openings or penetrations are formed in the lateralsides of the cask body. The wind can cause air pressure to be greater onthe windward side of the cask than the leeward side, thereby resultingin unbalanced and preferential flow rates of cooling air entering thecask. This can unevenly cool portions of the canister 101, particularlyif the hot spots on the SNF canister are on the leeward side of the caskwhich receives less cooling air. By contrast in the present vertical airinlet design of cask 100, however, the inlet plenum 151 from which thecooling air is drawn into the cask is naturally pressure balanced andthe air thoroughly mixed resulting in more uniform inlet air pressureand even cooling of all sides the canister.

Turning now to the upper end of the cask 100, the cooling air outlet 220of the ventilation system in lid 114 is formed by a plurality of outletducts 221 extending between upper annular member 114 a defining thecentral air outlet opening 114 f and lower disk member 114 b previouslydescribed herein. Outlet ducts 221 are in fluid communication with caskinternal cavity 121 at bottom and air outlet opening 114 f at top whichis in fluid communication with ambient atmosphere via a hollow tubulartop weather protection cap structure 225 which forms an upwardlyprotruding lid extension as shown. Cap structure 225 may comprise avertical cylindrical sidewall 226 fitted with a perforated vent screen227 on its upper portion and preferably solid top cover plate 228 toprevent the direct ingress of rain into the cap. Vent screen 227 mayhave an open area of approximately 50% in one non-limiting embodiment;however, other open areas may be used. An open interior 229 is definedinside cap structure 225 which is in fluid communication at bottom withthe air outlet opening 114 f in the lid upper annular member 114 a andlaterally/radially to ambient atmosphere through the vent screen 227.

Outlet ducts 221 in lid 114 have a circuitous shape in transverse crosssection such that no straight line of sight exists between cask internalcavity 121 and outlet opening 114 f or the ambient environment. Thisprevents straight line radiation streaming in the upward direction asexemplified by the flow path followed by the heated cooling airdescribed below.

A method for operation of the natural convective thermo-siphon airventilation system will now be summarized. FIG. 19 is a cross sectionalview of cask 100 showing the cooling air flow path therethrough andcorresponding directional flow arrows.

In operation, referring to FIG. 19 and other applicable figures, ambientcooling air is first drawn radially inwards into inlet plenum 151between standoff members 150 from the ambient environment a full 360degrees around the bottom of the cask 100. The air then mixes first andflows vertically upwards through air inlet openings 201 in baseplate 115of canister support structure 140 and enters the inlet ducts 202. Theair flows vertically upwards, then radially outwards, and finallyvertically upwards in ducts 202. The still cool ambient air is drawninto the bottom of ventilation annulus 122 of cask 100 from the annulararray of upward discharging duct exit openings 206 of the inlet ducts.

Canister 101 heats the incoming air in ventilation annulus 122 whichrises upwards and collects in an annular air outlet plenum 222 formedwithin cask internal cavity 121 between inner shell 112 and lower diskmember 114 b of the lid. The now heated cooling air flows verticallyupwards in the plenum and then turns radially inwards through outletduct 221 to central outlet opening 114 f in upper annular member 114 aof the lid. The heated cooling air continues to flow vertically upwardsinside cap structure 225 and then is discharged radially/laterallyoutwards through vent screens 227 to ambient atmosphere. Heating of theair in the ventilation annulus 122 by the fuel assemblies within thecanister 101 drives the foregoing air circulation continuously as longas heat is emitted by the canister.

According to another aspect of the nuclear waste storage system, the airinlet region of cask 100 may be supplemented where needed for increasedradiation blockage with photon attenuators. These may be perimetricallypositioned in spaces beneath the cask body 110 and only in locations ofpotential increased radiation levels such as along cask storage areaborders or fences in close proximity to the loaded casks containing SNFor other high level radioactive waste. FIG. 17 shows the underside of acertain embodiment of cask 100 including arcuately curved radiationphoton scattering attenuators 160. The attenuators provided are eachfixedly coupled between a pair of the radial plate outer standoffmembers 150 a in the outer array of standoffs on the bottom baseplate115 previously described herein. A pair of attenuators is shown in thisfigure; however, more or less can be used as needed. The attenuators 160are configured with open areas to allow the ambient cooling air to flowinwards into the bottom air inlet plenum 151 of the natural ventilationsystem towards the air inlet 200 in canister support structure 140. Inthe non-limiting illustrated embodiment, each attenuator 160 maycomprise an arcuately curved outer support bar 161 and optional innersupport bar 163 which may each be welded at their ends to opposingstandoff members 150 a welded in turn to the bottom of baseplate 115. Aplurality of vertically orientated flat deflector plates 162 are weldedto and arcuately spaced apart on the bars 161, 163 as shown. Plates 162may be optionally welded to baseplate 115 in additional if needed foradded stability and support. In some embodiments, the bottom surfaces ofthe outer support bars 161 may be flush with the bottom surfaces of theouter standoff members 150 a as shown so that the bars rest on theconcrete base pad 130 with the standoffs. This provides added supportfor the attenuators 160 and cask. In one embodiment, support bars 161,163 and deflector plates 162 may be formed of metal such as steel oranother suitable metal. As shown in FIG. 17 , attenuators 160 are onlylocated where required and not necessarily around the entire perimeterof the cask bottom.

FIG. 18 shows gridded radiation photon scattering duct attenuator 170installed in the air outlet 220 of the cask ventilation system. The ductattenuator 170 is formed by an orthogonally intersecting array of steelor other metallic flat duct deflector plates 171 which may be fittedinside central air outlet opening 114 f in lid 114. FIGS. 22-23 show asimilar gridded duct attenuator 170 installed in inlet ducts 202 ofcanister support structure 140 at both the air outlet 206 and air inlet201 of each duct.

Features and advantages of the present cask with ambient cooling airventilation system include but are not limited to the following. Thehigh-density concrete liners 113 and 142 of the cask sidewall andcanister support structure 140 respectively may comprise hematite (ironoxide compound Fe₂O₃) and/or other iron-containing aggregates whichsubstantially boost the thermal conductivity of the liners resulting ina significant rate of conduction heat transfer from the inside surfaceof the cask to the outside surface. The heat duty of the cask istherefore accordingly increased to dissipate heat emitted by thecanister 101. Cask 100 has superior radiation shielding performanceattributable to no lateral/radial or vertical streaming paths forradiation emanating from the nuclear fuel stored in the canister insidethe cask. This also creates the absence of any open intrusion path forsmall penetrant missiles that can reach the nuclear fuel inside thecask. Air inlet and outlet openings optionally equipped with duct photonattenuators 170 which act to further attenuate radiation therebydecreasing ambient levels of radiation. Any minute dose allowed from thebottom region of the cask may be further reduced by using arcuatelycurved photon attenuators 170 at the periphery of the air inlet plenum151 beneath the cask. The penetration for the air outlet in the top lidfeatures the “disk & donut” geometry previously described herein with alarge overlap in the planar horizontal direction at the cask body 110 tolid 114 interface such that there is no path for direct verticallyupwards streaming of radiation from the cask internal cavity holding thecanister to the ambient environment. The structurally robust concreteand steel disk part of the lid also serves to block any incoming missileor projectile. The width of the air inlet and outlet flow passages (e.g.ducts) can be optimized to meet the needed heat duty of cask 100 withoutpermitting excessive amounts of diffused radiation inside the cask fromreaching the ambient environment. The ventilated cask 100 is configuredand suited for sheltered or unsheltered deployment on the concretestorage or base pad 130. The cask can therefore be stored inside abuilding that has adequate ventilation, or in unsheltered storagefacilities out in open air to facilitate improved ventilation andcooling of the cask.

While the foregoing description and drawings represent some examplesystems, it will be understood that various additions, modifications andsubstitutions may be made therein without departing from the spirit andscope and range of equivalents of the accompanying claims. Inparticular, it will be clear to those skilled in the art that thepresent invention may be embodied in other forms, structures,arrangements, proportions, sizes, and with other elements, materials,and components, without departing from the spirit or essentialcharacteristics thereof. In addition, numerous variations in themethods/processes described herein may be made. One skilled in the artwill further appreciate that the invention may be used with manymodifications of structure, arrangement, proportions, sizes, materials,and components and otherwise, used in the practice of the invention,which are particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being defined by the appended claims andequivalents thereof, and not limited to the foregoing description orembodiments. Rather, the appended claims should be construed broadly, toinclude other variants and embodiments of the invention, which may bemade by those skilled in the art without departing from the scope andrange of equivalents of the invention.

1-25. (canceled)
 26. A method for operating a passively ventilated nuclear fuel storage system comprising: providing a cask comprising an internal cavity and an ambient air ventilation system in fluid communication with the internal cavity and atmosphere; inserting a canister containing nuclear waste emitting heat into the internal cavity of a cask and onto a canister support structure fixedly coupled to the cask; drawing ambient cooling air into an air inlet plenum formed beneath the cask; flowing the cooling air upwards from the air inlet plenum through the canister support structure into the internal cavity of the cask; heating the cooling air in the internal cavity of the cask; and discharging the heated cooling air to atmosphere through a lid coupled to a top of the cask.
 27. The method according to claim 26, wherein the cooling air is heated by the canister in a ventilation annulus formed between the cask and canister in the internal cavity.
 28. The method according to claim 26, wherein the cooling air flows vertically upwards into the ventilation annulus from the canister support structure.
 29. The method according to claim 26, wherein the cooling air flows into the internal cavity of the cask through plurality of air inlet ducts formed vertically through the canister support structure, and the cooling air flows outwards from the internal cavity through a plurality of air outlet ducts in the lid.
 30. The method according to claim 29, wherein the air inlet ducts and air outlet ducts are each arranged in a circumferentially-extending annular array in the canister support structure and lid, respectively.
 31. The method according to claim 30, wherein no straight line of sight exists through the air inlet and outlet ducts to prevent radiation streaming
 32. The method according to claim 26, wherein the drawing step comprises drawing air radially inwards into the air inlet plenum which is formed between a baseplate of the canister support structure and concrete base pad which supports the canister.
 33. The method according to claim 32, wherein the air inlet plenum is formed by a plurality of standoff members which elevate the baseplate apart from the base pad.
 34. The method according to claim 26, wherein there are no penetrations in a sidewall of the cask associated with the ventilation system. 